Ferguson 621 U woes. - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

GeoffK · 14th Aug 2020, 10:01 am

Well done on spotting R23 not being soldered. This is a 2.3Meg Ohm and like some of the other resistors may have gone high in value. It would have upset the grid bias to V2 as it appears to be in a feedback loop from the AM detector V4. Other causes of microphony can be if the tuner mounts have gone hard, causing vibration feedback from the speaker. But valves can go microphonic due to mechanical faults within the valve.

poppydog · 14th Aug 2020, 8:59 pm

Thanks for the reply Geoff. I had already looked for dry joints trying to sort out the rustling in the speaker but appeared to have missed the joint on r23, the tuning cap mounts are fine.
The big issue now is this hum thats being picked up on lw/mw, i dont get this on fm, its a echo type hum that can be heard very clearly and can be varied with the volume pot. I have tried putting one of those metal screening sleeves over the uch81 and even tried a replacement uch81 but to no avail.
I'm hoping somebody on here can come up with some ideas as once this hum is sorted its almost done

poppydog

Nuvistor · 14th Aug 2020, 9:40 pm

I may have missed this but is the 0.02 capacitor from the anode of the rectifier to chassis in good condition?
Thinking it could be modulation hum.
The other possible cause is interference from another source, fluorescent lights, switch mode power supplies etc.

poppydog · 15th Aug 2020, 12:29 pm

I could replace c60 with a 22nf and see if it makes any difference, is there a way to test this in circuit other than removing it to test? Its one of those wafer caps and looks in good physical condition.
If c60 was faulty would it not affect fm as well?
Re-soldering r23 seems to have made this hum very noticeable.
I have no other electrical items nearby that i think could be causing this but I will try it later in a different room.

poppydog

Nuvistor · 15th Aug 2020, 12:34 pm

By wafer I think it’s a ceramic, not likely to be faulty.
Modulation hum is less likely to problems on FM.

poppydog · 15th Aug 2020, 1:09 pm

Well I will stick a replacement in and if it makes no difference I will just have to live with it.
As this is more or less finished now the thread can be closed.
Thanks for all the help.

poppydog

14th Aug 2020, 10:01 am	#81
GeoffK Heptode Join Date: Mar 2016 Location: Essex, UK. Posts: 602	Re: Ferguson 621 U woes. Well done on spotting R23 not being soldered. This is a 2.3Meg Ohm and like some of the other resistors may have gone high in value. It would have upset the grid bias to V2 as it appears to be in a feedback loop from the AM detector V4. Other causes of microphony can be if the tuner mounts have gone hard, causing vibration feedback from the speaker. But valves can go microphonic due to mechanical faults within the valve. __________________ *Geoff*

14th Aug 2020, 8:59 pm	#82
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Ferguson 621 U woes. Thanks for the reply Geoff. I had already looked for dry joints trying to sort out the rustling in the speaker but appeared to have missed the joint on r23, the tuning cap mounts are fine. The big issue now is this hum thats being picked up on lw/mw, i dont get this on fm, its a echo type hum that can be heard very clearly and can be varied with the volume pot. I have tried putting one of those metal screening sleeves over the uch81 and even tried a replacement uch81 but to no avail. I'm hoping somebody on here can come up with some ideas as once this hum is sorted its almost done poppydog

14th Aug 2020, 9:40 pm	#83
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Ferguson 621 U woes. I may have missed this but is the 0.02 capacitor from the anode of the rectifier to chassis in good condition? Thinking it could be modulation hum. The other possible cause is interference from another source, fluorescent lights, switch mode power supplies etc. __________________ Frank

15th Aug 2020, 12:29 pm	#84
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Ferguson 621 U woes. I could replace c60 with a 22nf and see if it makes any difference, is there a way to test this in circuit other than removing it to test? Its one of those wafer caps and looks in good physical condition. If c60 was faulty would it not affect fm as well? Re-soldering r23 seems to have made this hum very noticeable. I have no other electrical items nearby that i think could be causing this but I will try it later in a different room. poppydog

15th Aug 2020, 12:34 pm	#85
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Ferguson 621 U woes. By wafer I think it’s a ceramic, not likely to be faulty. Modulation hum is less likely to problems on FM. __________________ Frank

15th Aug 2020, 1:09 pm	#86
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Ferguson 621 U woes. Well I will stick a replacement in and if it makes no difference I will just have to live with it. As this is more or less finished now the thread can be closed. Thanks for all the help. poppydog